Glow discharge starters comprise an hermetically sealed envelope containing an ionizable medium and a pair of normally spaced-apart electrodes, at least one of which is a thermally responsive bimetallic electrode. The pressure of the gas, the spacing of the electrodes and the characteristics of the other electrically conducting elements of the starter are selected so that on application of a predetermined voltage across the starter, known as the breakdown voltage, a glow discharge occurs between the starter electrodes.
Such glow discharge starters are used extensively for starting preheat fluorescent lamps and similar gaseous electrical discharge devices having electrodes which require heating prior to starting. The lamp circuit generally includes a ballast transformer which serves to limit the current through the lamp connected in its output circuit. The starter is connected in shunt with the discharge path through the lamp and in series with the electrodes; for instance, one terminal of each electrode of the lamp is connected to the output circuit of the ballast and the other terminal of each electrode is connected across the starter. At starting, the open circuit voltage of the ballast exceeds the breakdown voltage of the starter and starts a glow discharge therein which heats the thermally responsive electrode and causes it to engage the other electrode in the starter. The closure of the starter effectively short circuits the lamp electrodes in series across the ballast; thereupon the increased current flow through the electrodes raises them rapidly to an electrode emitting temperature. Simultaneously, the glow discharge through the starter is extinguished and the switch electrodes begin to cool. After a time, the thermally responsive electrode in the starter disengages itself from the stationary electrode and the heating circuit is thereby opened. A transient voltage surge or kick then occurs due to the inductance in the ballast. This surge impressed across the lamp normally starts the main discharge between the lamp electrodes; if the lamp fails to start, this cycle is repeated until starting is achieved. After the main discharge through the lamp has started, the voltage drop thereacross is less than the breakdown voltage of the starter so that the starter does not develop a glow discharge and its electrodes remain disengaged.
A glow discharge starter of the aforementioned type is shown and described, for example, in the book "IES Lighting Handbook, 1981 Reference Volume", pages 8-35 and 8-36.
Glow discharge starters intended for operation on a 110-120 volt commercial circuit require the addition of a quantity of a low electron work function material in order to lower the breakdown voltage of the glow discharge. Several methods are known for reducing the breakdown voltage. For example, U.S. Pat. No. 4,562,379, which issued to Dobashi et al, discloses the use of a film deposited on the inner surface of the envelope and formed of a mixture of metal barium and metal oxide. U.S. Pat. No. 2,740,861, which issued to Lake on Apr. 3, 1956, discloses coating the electrodes with zinc to lower the breakdown voltage of the glow discharge starter.
Some commercially available glow discharge starters employ a coating of a lanthanum-nickel alloy on one of the electrodes. The lanthanum-nickel alloy provides a low work function emissive material necessary to obtain the required breakdown voltage. The lanthanum also acts to getter contaminants, such as oxygen, from the fill gas. However, the lanthanum-nickel alloy, which is effective in providing the desired results, is very reactive to air and tends to oxidize at room temperature. Since the effect is greatly accelerated at the melting temperature achieved during the coating application of the molten alloy to the electrode (e.g., 485.degree. C.), the alloy must be maintained under an inert atmosphere, such as argon. Also, exposure of the lanthanum-nickel alloy to air must be kept to a minimum after the coating process. The freshly coated surfaces begin to oxidize immediately and thereby affecting the performance of the glow discharge starter.